Recently, optical communication testing instruments, such as the fiber optic time domain reflectometer (OTDR), have been used for evaluating various characteristics of an optical fiber. In the OTDR, an output pulse of infrared light is emitted from a receptacle provided on the front panel of the instrument. A plug with an optical fiber to be tested is coupled to the receptacle. It is well known that infrared light is harmful to the human eye. Accordingly, the operator of the instrument has to take care to avoid exposure of his eyes to the infrared light when the plug is not coupled to the receptacle. Some optical communication testing instruments have covers for the receptacles. However, it is troublesome to remove and replace the cover each time the plug is connected to and disconnected from the receptacle.